doc: Use correct line break to fix build

Escaping the backslash we want to use for a line break is a really bad idea.
Introduced in f126afbba2 to support a crippled
echo without new line option.

The linebreak was not detected and thus the first image name used as a target.
Broke our nightly efl build with make distcheck and made me really unhappy to
while searching for the cause.

doc/Makefile.am

@@ -54,7 +54,7 @@ include images.mk
 
 all-local:
 	@rm -f images.tmp; \
-	echo 'EXTRA_DIST += \\' > images.tmp; \
+	echo 'EXTRA_DIST += \' > images.tmp; \
 	(cd $(srcdir) && find ./img -type f -print | cut -d/ -f2- | grep -v '~' | sort | tr '\n' ' ') >> images.tmp && \
 	echo >> images.tmp && \
 	if ! cmp "$(srcdir)/images.mk" images.tmp >/dev/null; then \